If you are going to "discount", I would never do it by lowering the price. I would either lower the service by cutting out the hours worked, or by giving a print credit as someone else suggested. You can throw in a extra free 11X14 of any photo they buy at full price, for example. That costs you very little and if the condition is that they have already paid full price for the photo. If they get to have a DIFFERENT photo free, there is the opportunity cost of them not paying full price for it. I realize that some customers have budget restraints, but sometimes you just aren't a "match". Hold firm and if you must give a little, give a little that has huge perceived value.
. These are identical items that can be sourced from many different retail outlets.
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